Juan Zapata

Generalized-scattering-matrix analysis of a class of finite arrays of coupled antennas by using 3-D FEM and spherical mode expansion

A rigorous method for characterizing, in terms of a multimode scattering matrix, a finite array of antennas where each antenna can be described by means of spherical waves is presented in this paper. The procedure provides the impedance, coupling and radiating characteristics of the arrays and comprises two steps. First, the generalized scattering matrix (GSM) of each single antenna is numerically calculated over a wide band of frequencies. For this purpose we use a previously-developed methodology that combines the domain segmentation technique, the three-dimensional finite element method (3D-FEM), spherical mode expansion and a reduced order model obtained using a symmetric matrix Pade/spl acute/-Via-Lanczos (SyMPVL) algorithm. Second, the overall GSM of the finite array is calculated starting from the GSM of each antenna and using rotation and translation of spherical waves. A closed-form expression for the overall GSM is given and different examples are shown in order to validate the proposed method.

Ultrawideband Optimized Profile Monopole Antenna by Means of Simulated Annealing Algorithm and the Finite Element Method

Monopole antennas with an optimized profile are described. They are designed by means of a global optimization using the simulated annealing algorithm and the Finite Element Method. Two designs are presented. The first optimized profile monopole is designed to obtain the lowest return losses over the extremely high bandwidth that lies between 4 GHz and 20 GHz. Better than 17 dB return losses are obtained in all the bandwidth. This initial result is the basis for a more detailed work. The second antenna is designed to optimize return losses in the 2 to 11 GHz band. This frequency band has been chosen to cover the definition of ultrawideband (UWB). Return losses of less than 24 dB have been achieved in the whole band in the simulation step using an infinite ground plane. A prototype has been built, measured and resimulated. Return losses of less than 20 dB have been measured in the whole band using a finite ground plane. Results from the simulation of the prototype with the finite plane show an excellent agreement with measurements. A final parametric study of the optimization variables is carried out.

A CAD-oriented method to analyze and design radiating structures based on bodies of revolution by using finite elements and generalized scattering matrix

In this work, an efficient analysis of radiating structures based on bodies of revolution is dealt with. The procedure is a hybrid method based on a segmentation of the structure into two-dimensional regions, finite elements and a spherical computation domain surrounding the antenna, defining a boundary or port where a spherical mode expansion of the fields is used. A reduced order model is computed for a fast frequency sweep. To validate this method, some structures based on bodies of revolution as a rod dielectric antenna, conical dielectric-loaded horns, profiled horns and a monopole-dielectric resonator antenna are studied and results are compared with those demonstrated by other authors. The design of a smooth-walled horn by means of the optimization of the profile is also carried out

ANN Characterization of Multi-Layer Reflectarray Elements for Contoured-Beam Space Antennas in the Ku-Band

The analysis of a 1.2-meter, contour-shaped reflectarray antenna through the use of Artificial Neuronal Networks (ANNs) is carried out in this paper. The analysis is a two-step procedure: reflectarray element modeling and pattern synthesis. In the first step, artificial neural networks are found to reproduce both the amplitude and the phase of the complex reflection coefficient of the three-layered reflectarray element. For this task, up to 9 free input parameters are considered: six geometrical parameters, the incident angle in terms of azimuth, θ, and elevation, φ, and the frequency. Because of this large number of free parameters, a new artificial neural network training methodology has been developed regarding both the training set and the training process itself. In the second step, extensive full wave electromagnetic computation is replaced by trained artificial neural networks to calculate the electric field on the planar structure and the radiation pattern. A good agreement is obtained compared to an analogous analysis carried out by Method of Moments. Thanks to this methodology, the speed up factor in terms of time is in the order of 7×102, which represents a significant improvement in Computer Aided Design (CAD) of reflectarray antennas.

Antenna-Generalized Scattering Matrix in Terms of Equivalent Infinitesimal Dipoles: Application to Finite Array Problems

A full-wave antenna modeling method by means of elementary sources described in terms of colocalized infinitesimal dipoles is presented in this paper. The procedure provides a generalized scattering matrix (GSM) in terms of equivalent infinitesimal dipoles from the GSM of the antenna by using rotation and translation of spherical modes. The number of equivalent infinitesimal dipoles and their positions are optimized by using a genetic algorithm. The application to the full-wave analysis of arrays is also presented, providing very accurate results even when the minimum spheres of the antennas in the array environment overlap considerably, which cannot be solved by the direct coupling of generalized scattering matrices in terms of spherical waves. Some examples of antenna modeling and their application to the analysis of arrays of antennas on a metallic plane will be shown.

Spherical-Waves-Based Analysis of Arrays of Volumetric Antennas With Overlapping Minimum Spheres

Arrays of volumetric antennas whose minimum spheres overlap are efficiently analyzed by means of translational addition theorems for spherical modes in this letter. For this purpose, an improved model of the isolated antenna in terms of elementary sources of infinitesimal dipoles is used. The model provides a generalized scattering matrix (GSM) in terms of infinitesimal dipoles that allows synthesizing the whole behavior of the isolated antenna and enables its precise application to array environments by using translational addition theorems. The search region for the optimization procedure that finds the positions of the elementary sources to model the isolated antenna has been extended to the volume occupied by the antenna, so that the model becomes very precise. As a consequence, even when the minimum spheres of the array elements are strongly overlapped, it provides accurate results since the equivalent models will never overlap. Additionally, the optimization procedure is carried out only once for a wide band. The proposed approach has been validated by means of two different radiating elements such as monopoles and dielectric resonator antennas (DRAs), with good agreements obtained in comparison to the full-wave results simulated by commercial software.

A Fast Technique to Estimate the Mutual Coupling Coefficients From the Transmitting Characteristics of an Isolated Element

This letter presents a fast technique to estimate the mutual coupling between the elements in an antenna array from the transmitting characteristics of an isolated element. This approximation is obtained from the transmitting coefficients in terms of spherical waves in reciprocal antennas by using the translational addition theorems. The external coupling is interpreted as a contribution of all different orders of reflection between the array elements where the first order of coupling interactions does not depend on the scattering characteristics of the elements. This method is validated through the estimation of the mutual coupling between different elements compared to the full-wave response.

A comparison of scalar singular finite elements applied to the analysis of homogeneous waveguides with sharp edges

When we solve the wave equation by the finite element method, the order of convergence and the accuracy of the solution are reduced by the presence of sharp edges. The employment of singular elements improves the solution and allows us to reduce the cost of computation. In this paper, three types of scalar singular finite elements with the capacity to handle singularities in the derivative, and which have previously been applied to mechanical problems, are examined, together with standard elements, in the context of homogeneous waveguide analysis. We solve two examples of homogeneous waveguides showing the different behaviour of the singular elements and obtaining information on the order of convergence and the approximation of the gradient of the unknown function.

On the higher-order approximations for efficient computational electromagnetics

In this work we propose an efficient procedure to obtain Loop and Star basis functions useful to the Surface Integral Equation. They are of any order, for 3D curved surfaces, and they keep the solenoidal/non-solenoidal splitting, improving the performance for low-frequency (near field). The approach incorporates a coordinate transformation to cancel out the weak singularity of Electric Field Integral Equation (EFIE) which is also capable to cope with the issue of the nearly singularities.

Planar ESPAR Array Design with Nonsymmetrical Pattern by Means of Finite-Element Method, Domain Decomposition, and Spherical Wave Expansion

The application of a 3D domain decomposition finite-element and spherical mode expansion for the design of planar ESPAR (electronically steerable passive array radiator) made with probe-fed circular microstrip patches is presented in this work. A global generalized scattering matrix (GSM) in terms of spherical modes is obtained analytically from the GSM of the isolated patches by using rotation and translation properties of spherical waves. The whole behaviour of the array is characterized including all the mutual coupling effects between its elements. This procedure has been firstly validated by analyzing an array of monopoles on a ground plane, and then it has been applied to synthesize a prescribed radiation pattern optimizing the reactive loads connected to the feeding ports of the array of circular patches by means of a genetic algorithm.